Scan Cycle Consists of 3 steps:-
Input Scan
PLC
takes a snapshot of the input and solves the logic the plc looks at each input
card to determine if it is ON or OFF and saves the information in data table
for use in next step.
Execute Program (Logic Execution)
PLC executes a program one instruction at a time using only the memory copy of
the Inputs
Output Scan
When the
ladder scan completes , the outputs are updated using temporary values in
memory . The PLC updates the status of the outputs based on which inputs
were ON during the first step and the results of executing a program during the
second step.
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